Video signal compression apparatus typically include circuitry to quantize codewords of partially compressed signal, causing many of the quantized codewords to have similar values, which condition is conducive to run length encoding. Thereafter the quantized codewords are typically run and variable length encoded to provide the signal in compressed format. The term "partially compressed signal" refers to signal provided by, for example, a discrete cosine transform, or DCT, processor. Video data provided by DCT processors occurs in blocks of, for example, 8.times.8 coefficients representing a matrix of 8.times.8 contiguous pixels. Typically the respective coefficients in a block are quantized by different amounts depending upon the relative importance of the respective coefficients. In quantizing a block of such coefficients a quantization matrix is utilized. This quantization matrix includes the quantizing values to be applied to the respective coefficients in a block. Quantization of the video data may be performed by applying the same matrix or table of quantization values to all blocks of coefficients (codewords). This is referred to herein as fixed quantization, even though the quantizing values in the matrix may be different. Since different frames of video signal represent different images, it will be readily appreciated that the amount of compressed video data resulting from fixed quantization will be variable from frame to frame, giving rise to variable bit rate, VBR, compression.
Certain signal transmission systems cannot efficiently accommodate VBR compressed signal, hence constant bit rate, CBR, compression systems have been developed. In the CBR systems the quantization of codewords is made adaptive to control the number of compressed data. Typically the volume of compressed data is monitored and the quantization values are changed in inverse proportion to the volume of compressed data in order to render the number of compressed bits per frame substantially constant. Adaptive quantizing systems also utilize tables of quantizing values for quantizing the blocks of coefficients. Adaptivity is typically realized by simply scaling the quantizing values as they are output from the table for application to the respective coefficients of a block. Nominally all quantizing values provided from the quantization table are scaled by the same factor, but the scale factor may vary from block to block.
CBR compression systems tend to cause the image quality from frame to frame to vary., but constant bit rate is advantageous if the compressed signal is to be multiplexed with other signals for transmission. Alternatively, VBR compressed signal tends to provide substantially uniform image quality from frame to frame, but is more difficult to transmit in a multiplexed signal situation.